As one method for forming an electroconductive circuit on a printed wiring board, an electroconductive powder such as gold, silver, copper, or carbon is used, and this is mixed with a binder, an organic solvent, and, as necessary, an additive, etc. to form a paste (e.g., Denshi Zairyo (Electronic Materials), Oct. 1994, pp. 42 to 46). In particular, in a field in which high electroconductivity is required, a gold powder, a silver powder, a palladium powder, or an alloy powder thereof is generally used.
Among the above-mentioned pastes, because it has good electroconductivity an electroconductive paste containing a silver powder is used for formation of a printed wiring board, a wiring layer (an electroconductive layer) of an electronic component, etc., or formation of an electric circuit or an electrode of an electronic component, but when an electric field is applied thereto under conditions of high temperature and high humidity, electrodeposition of silver, which is called migration, occurs on the electric circuit or the electrode, thus causing a short circuit between electrodes or between wiring, which is a drawback. Several countermeasures against this migration have been taken, for example, coating the surface of a conductor with a moisture-proof coating or adding a corrosion inhibitor such as a nitrogen-containing compound to the electroconductive paste, but none thereof had a sufficient effect. Although migration resistance can be improved by use of a silver-palladium alloy powder instead of the silver powder, since silver and palladium are expensive, the silver-palladium alloy powder is also expensive, which is a drawback.
Furthermore, in order to obtain a conductor having favorable resistance, it is necessary to increase the amount of silver powder added, and since the silver powder is expensive, the electroconductive paste is correspondingly expensive, which is a drawback. Use of a silver-coated copper powder can improve the migration and also give an inexpensive electroconductive paste. However, when the surface of a copper powder is coated uniformly and thickly with silver, the effect in improving the migration is not sufficient. Moreover, a coating obtained from the electroconductive paste cannot be subjected to direct soldering, which is a drawback. Furthermore, when the electroconductive paste employing a silver powder is subjected to soldering, silver erosion occurs and a good joint cannot be obtained, which is a drawback.
On the other hand, a copper powder is used in some cases instead of the silver powder. However, after an electroconductive paste employing a copper powder is heated and cured, the copper is very susceptible to oxidation, and the copper powder reacts with oxygen contained in air or the binder, thus forming an oxide film on the surface thereof and thereby greatly degrading the electroconductivity. In order to overcome this, a copper paste having its electroconductivity stabilized by adding various types of reducing agent so as to prevent the oxidation of the surface of the copper powder has been disclosed, but the electroconductivity and the stability of the electroconductivity thereof are inferior to those of the silver paste, and the resistance thereof increases in a high temperature, high humidity test, etc., which is a drawback.
Furthermore, unless the content of the copper powder in the electroconductive paste is high, stable electroconductivity cannot be obtained. However, when the content of the copper powder is high, the adhesion might be degraded or the storage stability might be poor, which is a drawback. Moreover, the coating of the copper paste thus obtained cannot, with a conventional copper paste, be directly soldered onto, which is also a drawback.
Conventionally, when a known electroconductive paste is used as an adhesive, since electroconductive powders are expensive, the electroconductive paste is also expensive compared with a solder paste, which is a drawback. There is therefore a desire for an electroconductive adhesive that has a more reliable electroconductivity than that of the copper paste, has better migration resistance than that of the silver paste, and gives excellent workability for solder paste and for drying/curing.
Furthermore, since conventionally known electroconductive pastes cannot have solder directly applied thereon, a coating of the electroconductive paste is subjected to an activation treatment before carrying out electroless plating, or copper electroplating is carried out in a plating solution using the coating as a cathode and the copper coating obtained by plating is subsequently subjected to soldering. However, unless the bonding between the coating and the copper plating is reliable, it is not practical. Therefore, if an electroconductive paste that can have solder applied thereon without carrying out electroless plating or electroplating is developed, the circuit formation step can be greatly shortened, which is a major advantage.
A solder is easily joined to a metal but does not join to a binder. When soldering is carried out, a coating is ideally formed from an electroconductive powder alone, which is then subjected to soldering, but there is the problem that no coating can be formed from an electroconductive powder alone without a binder.
A binder is therefore used to form an electroconductive paste. However, there is a restriction on the amount of binder in order to achieve reliability and workability of coating formation; for example, if the proportion of the binder is high, the electroconductive powder, which is a metal, is covered with the binder, there can be no areas where the solder and the electroconductive powder are in contact with each other, the solder cannot therefore adhere, and the electroconductivity is degraded, which are drawbacks.
In order to form an electroconductive paste to which a solder can adhere, the composition should be as close as possible to that of a copper foil. That is, it is ideal for the paste to have a composition such that when the electroconductive powder is placed in a given space, the electroconductive powder is highly packed, and the binder occupies only the volume corresponding to gaps between the electroconductive powder particles.
However, when the proportion of the electroconductive powder is made high as described above, the viscosity of the electroconductive paste becomes extremely high, and it becomes difficult to prepare an electroconductive paste, the workability when applying the electroconductive paste is degraded, and since the amount of binder for combining the electroconductive powder particles is small, the strength of the coating is also degraded. Furthermore, when the paste is used as an electroconductive adhesive, since the adhesion is low, it is not suitable for that use. Moreover, when solder joining is carried out using the electroconductive paste, it is necessary to use an electroconductive paste having a good balance between soldering properties, electroconductivity, workability, strength, and cost.
When the electroconductive paste is used as a solder substitute material for the purpose of electroconductive adhesion, in addition to printability, adhesion, and reliable conductivity, the workability, that is, the paste being dried and cured in a short time, is also important. When drying and curing a solder substitute adhesive, it is preferable to use a reflow oven, which has been used by assembly manufacturers for soldering chip components, from the viewpoint of effective utilization of equipment. In the case of general silver pastes, there is the defect that drying and curing at high temperature for a short time in a solder reflow oven easily causes swelling. Copper pastes also have the defects that curing at high temperature for a short time does not give stable electroconductivity, and in reliability tests such as a constant temperature, constant humidity test and a gas-phase cooling and heating test, the so-called open circuit, that is, loss of conduction, is observed.
An electroconductive paste is used in a method for forming an electroconductive layer as shown in FIG. 1 by dispersing an electroconductive powder in a binder to form an electroconductive paste, and coating the surface of a substrate with the electroconductive paste or filling a through hole therewith. In FIG. 1, 1 denotes the electroconductive paste and 2 denotes a copper foil.
With regard to another means for forming an electroconductive layer in a through hole formed in a printed wiring board, there is a method for forming an electroconductive layer by subjecting the inner wall of the through hole to copper plating.
When a hole-filling electroconductive paste for filling a through hole is employed connection between layers generally requires high electroconductivity even though the hole is small, and it is therefore necessary to pack the hole with the electroconductive paste as tightly as possible, and embed the electroconductive paste in the hole without any gaps. Conventional hole-filling electroconductive pastes are therefore required to have a high proportion of electroconductive powder, but when the proportion of electroconductive powder is made high, the viscosity of the electroconductive paste increases, and the hole packing properties thereof are degraded. Conversely, when the proportion of binder is made high, the viscosity decreases and the hole packing properties are therefore improved, but the electroconductivity is degraded, which is a drawback.
As a countermeasure thereagainst, a solvent-free type of electroconductive paste containing no solvent and employing as a main component of the binder a liquid epoxy resin is used, or an electroconductive paste is used that contains, depending on the hole size, a small amount of solvent.
However, since the epoxy resin shrinks less due to heat curing than a phenolic resin does, it is difficult to lower the resistance of the electroconductive paste containing the epoxy resin as a main component, which is a drawback.
Although the resistance can be reduced by increasing the proportion of electroconductive powder in the electroconductive paste or by using a metal powder having high electroconductivity such as silver, the resulting electroconductive paste is expensive.
On the other hand, there is an electroconductive paste containing a phenolic resin as a main component, and although this electroconductive paste has better electroconductivity than that of the electroconductive paste containing the epoxy resin as a main component, the viscosity of the electroconductive paste is high and there is a problem in terms of the hole packing properties.
When an electroconductive layer is formed within a through hole using the electroconductive paste, if the electroconductive paste used for filling the through hole contains a large amount of solvent, voids are inevitably formed within the through hole during drying of the solvent. The voids are a drawback for a multilayer circuit board as shown in FIG. 2 in which an insulating layer 5 is formed on the surface of a substrate 3, an end portion of the through hole filled with the electroconductive paste, a copper foil land 7, and part of a copper foil circuit 8, and an electroconductive material printed circuit (hereinafter, called a printed circuit) is further formed on the insulating layer 5 using an electroconductive material (jumper electroconductive paste), and it is necessary to eliminate any voids within a through hole 10, thereby improving the connection reliability between the through hole 10, the copper foil land 7, the copper foil circuit 8, and the printed circuit. In FIG. 2, 4 denotes an electroconductive layer, 6 denotes a jumper circuit, and 9 denotes an overcoat layer.
When a multilayer circuit board is fabricated by making a through hole conduct by means of copper plating formed on the inner wall of the through hole, after the inner wall of the through hole is subjected to copper plating, by applying cap plating over the electroconductive paste with which the through hole is filled, the above-mentioned drawbacks can be eliminated, but the number of steps increases and the cost also increases, which is undesirable.
There is also a method in which the inner wall of a through hole is subjected to copper plating so as to form an electroconductive layer, and the cavity is filled with a resin, and this method also has the defect that the number of steps increases and the cost is thus high.
There is also a method in which a through hole is filled with a voidless or substantially voidless electroconductive material so as to ensure conduction of the through hole, and an insulating layer and a printed circuit are then formed on the surface of a substrate. In this method, since the electroconductive material with which the through hole is filled and a copper foil land portion are connected to each other via the cross section of an end portion of the copper foil, there is the defect that the reliability of the connection is low. In order to eliminate this defect, the above-mentioned cap plating may be carried out, but this increases the number of steps and the cost, which is undesirable.
When a multilayer circuit board is fabricated by using a silver through hole wiring board in which a through hole is filled with a silver electroconductive material (silver paste) containing a solvent at 15 wt % or more and forming an insulating layer and a printed circuit on the surface of this wiring board, a large cavity is formed within the through hole accompanying evaporation of the solvent, resulting in a decrease in the reliability. That is, when an ionic impurity remains within the void during a washing step, etc., the migration resistance is degraded. Furthermore, in the case of the silver through hole wiring board, the silver paste might be thickly built up on a copper foil land, and the height of this thick, built up silver paste might obstruct the mounting of components.
On the other hand, there is a soldering material containing lead as a main component, and such a soldering material has been widely put into practice for a long time because it has a comparatively low melting point and good workability.
However, recently, restrictions on the use of lead have been proposed since lead is highly toxic and the human body and the ecosystem might be easily affected when a lead-containing effluent is treated. At present, a low melting point metal brazing material employing, as a substitute for lead, a metal material having a comparatively low melting point such as bismuth is being developed, but since the melting point thereof is higher than that of lead solder, it is necessary to increase the heat resistance of the substrate material, electronic components to be mounted, etc., giving rise to the defects of technical difficulty, an increase in cost, etc.
A multilayer lamination step employing a standard hole-filling electroconductive paste involves filling a hole with the electroconductive paste, laminating a pre-dried build-up layer, and applying heat and pressure as the main drying. It is therefore necessary that the electroconductive paste is cured after the main drying, and it is also necessary that the electroconductivity is improved by the application of pressure after lamination compared with a case where pressure is not applied.
However, the conventional hole-filling electroconductive paste employs an epoxy resin as a main component of the binder, and generally uses an imidazole as a curing agent therefor, and when a substantially spherical silver-coated copper powder that has been subjected to a dispersion treatment to break up aggregates and has copper exposed on the surface thereof is used as an electroconductive powder, the curing properties of the electroconductive paste might be degraded, which is a drawback.
When a silver-coated copper powder that has been subjected to a dispersion treatment is used, it is necessary to add a material that does not form a chelating bond with copper and functions as a curing agent for an epoxy resin.
Furthermore, since the substantially spherical silver-coated copper powder easily aggregates in a silver plating step and has a low tap density, if a large proportion thereof is added to the electroconductive paste, the viscosity of the paste is undesirably increased.
Moreover, when the substantially spherical silver-coated copper powder that has been subjected to the dispersion treatment is used, since a resol type phenolic resin forms a chelating bond with the copper, the viscosity of the electroconductive paste increases during storage, which is a drawback.
Furthermore, when an electroconductive adhesive (electroconductive paste) is prepared using an alkoxy group-containing resol type phenolic resin and an epoxy resin as binders, if a problem occurs in a component that is bonded to a printed wiring board and the bonded component is replaced, it is necessary to heat the cured thermosetting resins to a high temperature so that they are in a rubbery state, and such a defect can be eliminated by the use of a thermoplastic resin as the binder.
The present invention provides an electroconductive paste that can contain a high proportion of an electroconductive powder, has excellent electroconductivity reliability and migration resistance, has a highly competitive price due to a reduced amount of silver plating, and is suitable for use in solder electrode formation, an electroconductive adhesive, etc.
Furthermore, the present invention provides an electroconductive paste that has good packing properties and excellent flowability.
Moreover, the present invention provides an electroconductive paste that has excellent shelf life, can be cured in a short time, has excellent short-time drying and curing properties when using an infrared oven (hereinafter, called an IR oven), and is suitable for forming a wiring board circuit, filling a hole, etc.
Furthermore, the present invention provides an electroconductive paste that can give low viscosity and a high degree of packing, and has good heat resistance since the epoxy equivalent is small.
Moreover, the present invention provide an electroconductive paste that has a stable shelf life.
Furthermore, the present invention provides an electroconductive paste that has suppressed spreading during drying after printing.
Moreover, the present invention provides an electroconductive paste that has, in particular, excellent curing properties.
Furthermore, the present invention provides an electroconductive paste that has an excellent shelf life and is suitable as an electroconductive adhesive that enables adhered components to be easily removed.
Moreover, the present invention provides an electroconductive paste that has good electroconductivity and a stable shelf life.
Furthermore, the present invention provides an electroconductive paste that has suppressed spreading during drying after printing, and excellent adhesion and flexibility.
Moreover, the present invention provides an electroconductive paste that has few voids under fast curing conditions using a reflow oven, has excellent adhesion, electroconductivity and printability, and is suitable for mounting a semiconductor device, a passive component, etc.